In pipelines and boreholes having lengths of several km there is usually a need for conveying down different equipment and tools and/or collecting or acquisition of measured data or samples, etc. For those purposes pulling tools or well tractors having different embodiments are being used, wherein some have wheels or chains providing for axial rolling of the pulling tool or well tractor on the pipe or borehole wall. The roller(s) or the chain(s) is(are) being pressed against the pipe or borehole wall with a force that is sufficient for achieving the desired axial propulsive force in a number of varying frictional conditions. The power supply is commonly effected via a cable connection to the surface.
Most known pulling tools or well tractors utilize electric/hydraulic operation(s). This means that an electric motor drives a hydraulic pump, which again supplies power to the hydraulic motor(s) in the driving wheel(s). Such a system will be technically complex, and consequently low efficiency will be achieved. With a limited supply of power through long cable(s), the traction or propulsion force will be substantially limited. In several operations great tractive or propulsive force or power is desirable.